The present invention relates to an assembly for positioning a heater in relation to a roll. The invention further relates to a press device with such an assembly.
The invention is particularly advantageously implemented in connection with impulse pressing of fibre webs in a so called shoe presses having an extended nip, but can also be implemented when positioning heaters in connection with other types of heated rolls in a paper machine.
In connection with the manufacture of wet formed fibre webs, for example paper webs, usually some kind of press device is used in order to increase the dry content of the fibre web by means of a mechanical dewatering before a subsequent drying process. The reason why pressing advantageously is utilised instead of drying is that it is more energy efficient, and that the pressing often plays an important role for the physical properties of the finished paper sheet or fibre material.
One of the more simple previously known types of press devices consists of two rotatable press rolls which can be brought into pressing contact with each other in order to form a nip through which the web fibre web is guided. Thereby, the wet fibre web is usually guided through the nip supported on a press felt, or between two press felts or press fabrics.
When for example the maximum achievable dryness increase of a web fibre web is concerned, also press devices having an extended nip have been developed in order to further increase the efficiency of the pressing. Press devices having an extended nip have also proved to provide greater possibilities of influencing, or of avoiding a too large influence, on the physical properties of the finished paper sheet of fibre material.
The evolution of the dewatering technique has also resulted in so called impulse technique, usually referred to as impulse pressing or impulse drying, which in principle implies that the pressing takes place at a strongly increased temperature. Trials have proved that even higher dry contents can be achieved after the pressing by means of utilising press devices which are based on impulse technique. Furthermore, the impulse technique has proved to provide further possibilities to influence the physical properties of the finished paper sheet or fibre material.
Press devices utilising a combination of an extended nip and impulse technique are also previously known. Accordingly, for example the patent publication U.S. Pat. No. 4,738,752 discloses an apparatus for removing fluid from a fibrous web. The disclosed apparatus is claimed to comprise a press member and a blanket cooperating with the press member for defining therebetween an elongated pressing section in such a way that the web is pressed between the press member and the blanket during the passage through the pressing section. The apparatus further comprises a concave press shoe for urging said blanket towards said press member so that fluid is pressed out from the web when the web passes through the pressing section.
The apparatus disclosed in U.S. Pat. No. 4,738,752 further comprises a heating means adjacent to the press member for transferring heat to the web, whereby the web is subjected for an extended period to increased pressure and increased temperature when it passes through the pressing section, so that the water vapour which is generated within the pressing section during the passage forces the fluid away from the web. The apparatus further comprises a thermal transfer means cooperating with the blanket for defining the pressing section therebetween, wherein the transfer means transfers heat from the heating means to the web during the passage of the web through the pressing section. Finally, the apparatus comprises a further blanket between the thermal transfer means and a convex surface defined by the press member, so that the blanket, the web, the thermal transfer means and the further blanket move together between the convex surface and a cooperating concave surface defined by said concave press shoe for removing fluid from the web.
According to U.S. Pat. No. 4,738,752, the convex surface can be provided by a rotatable press roll, wherein the heating means can be arranged adjacently to the rotatable press roll for heating it, resulting in a subsequent heating of the web which is to be pressed. The heating means can be of a number of different types, such as an induction heater, an infrared heater, a microwave heater, a burner, a resistance heater, a laser heater or the like.
However, it has been found that heating of for example a press roll of the above-mentioned type by means of an external heater can be associated with certain problems. When heating a shell of a roll, an uneven temperature profile may occur in the shell in the axial direction of the roll, i.e. transversely to the machine direction. Possible reasons for such an uneven temperature profile are, for example, that the distance from the heater to the shell surface varies in different positions along the axial direction of the roll, or that the fibre web is drier or more wet in streaks in different positions along the cross direction of the web after the forming. Because of the intrinsic thermal expansion properties of the roll shell, the roll shell will exhibit a larger thermal expansion in its radial direction in connection with streaks having a higher temperature, and a smaller thermal expansion in connection with streaks having a lower temperature. The larger thermal expansion in the radial direction in connection with streaks having a higher temperature will in turn bring the shell surface closer to the heater so that the temperature of the shell surface in the streak in question is increased even further, which in turn results in an even larger radial thermal expansion with a resulting local temperature increase, and so on. This xe2x80x9cvicious circlexe2x80x9d results in a fibre web which is unevenly dewatered/dried in the cross direction, and in the roll shell being subjected to very large thermal and mechanical stresses.
Therefore, the first object of the present invention is to provide an assembly for positioning a heater in relation to a roll, which eliminates the risk of a xe2x80x9cvicious circlexe2x80x9d of the above-mentioned kind being created.
In accordance with one preferred embodiment of the invention, the first object is achieved by means of a roll of the assembly having a substantially cylindrical shape with a shell surface, rotatable around a shell axis, at a first distance from the shell axis, the heater comprising at least one heater section intended for heating the shell surface, and the heater section being supported, via a suspension attachment, by a supporting means at a variable, second distance from the shell axis. Thereby, according to the invention, the heater section is provided with at least one distance control means arranged for being in contact with the shell surface during the heating and for automatically regulating the second distance by means of a self-regulating mechanism.
A second object of the present invention is to provide a press device, intended for pressing a fibre web in a nip, which utilises the assembly according to the invention.
In accordance with another embodiment of the invention, this second object is achieved by means of the press device comprising at least one roll having a substantially cylindrical shape with a shell surface rotatably arranged around a shell axis, and at least one heater intended to heat the shell surface before the nip during the pressing, wherein at least one assembly according to the invention is arranged for positioning the heater in relation to the roll during the pressing.
Further objects of the present invention will become evident from the following description, while the features enabling the further objects to be achieved are listed in the attached, dependent claims.